This invention relates to improved electrolytic capacitors and to electrolytes for use in such capacitors.
Recently, there has been an increase in the gain (capacity per geometric area) of the anode aluminum foil employed in electrolytic capacitors. Such an increase frequently will cause the equivalent series resistance (ESR) of a capacitor to exceed the design limit. In order to avoid this problem, it has become necessary to provide electrolytes for said capacitors having a very low resistivity. It is known that by use of dimethylformamide (DMF) as a solvent, electrolytes of relatively low resistivity (60 to 120 ohm cm at 30.degree. C.) may be obtained. Also electrolytes of low resistivity ma be made using dimethylacetamide (DMAC) as the primary solvent.
However, both these solvents suffer from the disadvantage of being highly toxic, they both have low TLV values and both absorb through the skin.
Another approach has been to employ, as the main solvent, gamma butyrolactone. Electrolytes employing this solvent can exhibit a low resistivity. However this solvent suffers from the defect of being relatively expensive. Additionally, this solvent (gamma dutyrolactone) does not wet the paper fibers of the insulating spaces employed in electrolytic capacitors to the same extent as do most of the other capacitor solvents. As a result, it is necessary to use the more expensive manila fiber paper in order to ensure that a sufficient amount of the electrolyte is present so that the low resistivity of the electrolyte results in a capacitor having a low ESR.
Niwa et al, U.S. Pat. No. 3,812,039, shows an electrolyte for an electrolytic capacitor in which the solvent employed is the safe and relatively inexpensive solvent--N-methylformamide. The electrolyte shown therein contains, as a solute, a monoammonium, monoamine or monoalkali metal salt of isophthalic acid or maleic acid. However when the electrical electrolyte contains a monosalt of isophthalic acid the specific resistance of the electrolyte is too high for many purposes (250 ohm.cm at 25.degree. C. Table 1).
When the solute employed is a monosalt of maleic acid the electrolyte exhibits a relatively low specific resistance at 25.degree. C., 72 ohm.cm. However, it has been found that capacitors containing as an electrolyte a solution of a monosalt of maleate in N-methylformamide develops an undesirably high ESR when operated at high temperatures--85.degree. C. and above.
U.K. Patent Specification 1269010 also shows an electrolyte for an electrolytic capacitor employing as a major solvent--N-methylformamide. However the solute employed is a dangerous and highly toxic salt of a polynitrophenol.
Chemical Abstracts 112-228252a discloses an electrolyte for an electrolytic capacitor containing N-methylformamide and diethylamine isophthalate in which the molar ratio of the amine to the isophthalic acid is 1:1. This electrolyte shows a higher resistivity than is desired, 160 ohm cm.